A uranium plant in Oak Ridge, Tenn., was once a secret guarded so fervently that the plant didn’t have an address, nor did it appear on any maps. But it’s a secret no more — in fact, it’s a tourist attraction. Parts of the plant are now open for visitors to explore on a 2.5-hour guided tour. But there’s a catch — the visitors must be American citizens.

The Oak Ridge facility is the site of the world’s first fully-operational nuclear reactor, and oversaw the separation of uranium 235 from natural uranium. It was also a key part of the project that instrumented the dropping of the atomic bombs over Hiroshima. The lives of those who lived and worked in Oak Ridge were closely intertwined with this secretive operation, so closely guarded that the word uranium was rarely used — tuballoy was a common stand-in.